Narrated to us Aḥmad b. Ziyād b. Ja`far al-Hamadānī[1] said `Alī b. Ibrāhīm b. Hāshim[2] from his father[3] from Ibn Abī `Umayr[4] from Abān (b. `Uthmān)[5] from Abī `Abd Allāh (عليه السلام) said, “Whoever fasts three days of Sha`ban, jannah iswājib for him, and the Messenger of Allāh will do shafā`ah on the day of Judgment for him”[6]

But then you could commit suicide and go straight to hell too right, irrespective of how many times you fasted the 3 fasts in Sha`baan because the authentic hadīth is clear... Whoever kills themselves will go to hell.

Brother we don't know how Allāh's mercy is... Allāh could be wiping away your major sins and you don't even know it... That's up to Allāh... Those who don't have major sins will go to heaven.

So you have to apply these sorts of ahādīth in your life and don't question them.

I believe you are not looking at this ḥadīth in a holistic manner. You are focusing on just this ḥadīth alone, instead you should see this ḥadīth along with the massive amount of other ḥadīth we have.

We have umpteen ḥadīth that says if you do not do your obligations (i.e. Prayers, wājib fasts, Hajj, etc.), then you will not enter heaven. Some ḥadīth even say that you are not considered a Muslim, if you do not do your Prayers, Hajj, etc.

The obligations of Allāh (عَزَّ وَ جَلَّ) come first, and when you have completed and done your obligations to Allāh (عَزَّ وَ جَلَّ), these mustaḥab actions is what puts your over the top. We will all have our deeds put on a scale, and small minute things as this (i.e. fasting three days in Sha`bān) can tilt your scale in the right direction.